Rare-earth magnets containing rare-earth elements such as lanthanoide are called permanent magnets as well, and are used for motors making up a hard disk and a MRI as well as for driving motors for hybrid vehicles, electric vehicles and the like.
Indexes for magnet performance of such rare-earth magnets include remanence (residual flux density) and a coercive force. Meanwhile, as the amount of heat generated at a motor increases because of the trend to more compact motors and higher current density, rare-earth magnets included in the motors also are required to have improved heat resistance, and one of important research challenges in the relating technical field is how to keep magnetic characteristics of a magnet at high temperatures.
Rare-earth magnets include typical sintered magnets including crystalline grains (main phase) of about 3 to 5 μm in scale making up the structure and nano-crystalline magnets including finer crystalline grains of about 50 nm to 300 nm in nano-scale. Among them, nano-crystalline magnets capable of decreasing the amount of expensive heavy rare-earth elements to be added while making the crystalline grains finer attract attention currently.
The following briefly describes one example of the method for manufacturing a rare-earth magnet. For instance, Nd—Fe—B molten metal is solidified rapidly to be fine powder, while pressing-forming the fine powder to be a compact. Hot deformation processing is then performed to this compact to give magnetic anisotropy thereto to prepare a rare-earth magnet (orientational magnet).
Various techniques have been disclosed for this hot deformation processing. In typical hot deformation processing, upsetting is performed, in which a compact (bulk) obtained by shaping magnetic powder is placed into a die, and pressure is applied to the compact with punches. Such upsetting, however, has a big problem that cracks (including micro-cracks) are generated at the outermost periphery of the rare-earth magnet processed where tensile stress is generated. That is, in the case of upsetting, the periphery part hangs over due to the friction acting on the end face of the rare-earth magnet, which causes such tensile stress. A Nd—Fe—B rare-earth magnet has weak tensile strength against this tensile stress, and so it is difficult for such a magnet to suppress the cracks due to such tensile stress. For instance, such cracks may be generated when the processing ratio is about 40 to 50%. The distribution of strains is equivalent to the non-uniformity of remanence (Br), and especially remanence is extremely low at a strain region of 50% or less, meaning that the material yield is low. To solve these problems, frictional resistance may be decreased, but a conventional method in which hot lubrication is performed depends on fluid lubrication only, and so it is difficult to use such a method for upsetting using an open system.
Such cracks generated at a rare-earth magnet cause the processing deformation that is formed to improve the degree of orientation to be open at the positions of the cracks, thus failing to direct the deformation energy to the crystalline orientation sufficiently. This becomes a factor to inhibit the improvement in remanence.
Then to solve the problem of cracks generated during upsetting as stated above, Patent Literatures 1 to 5 disclose techniques, in which a compact as a whole is enclosed into a metal capsule, followed by hot deformation processing while pressing this metal capsule with upper and lower punches. According to these techniques, they say that magnetic anisotropy of the rare-earth magnet can be improved while suppressing cracks that are a problem during hot deformation processing.
Although they say that the techniques disclosed in Patent Literatures 1 to 5 can solve cracks, it is known that such a method of enclosing the compact in a metal capsule causes the rare-earth magnet obtained by the hot deformation processing to receive strong constraints from the metal capsule due to a difference in thermal expansion during cooling and so generate cracks. In this way, cracks will be generated when a metal capsule is used as well, and to avoid such a problem, Patent Literature 6 discloses a method of making a metal capsule thinner by upsetting through multiple steps, so as to decrease the constraints from the metal capsule. For instance, Patent Literature 6 discloses the embodiment, in which an iron plate of 7 mm or more in thickness is used. Such an iron plate of 7 mm or more in thickness, however, cannot be said thin enough to prevent cracks completely, and it is known that cracks generate actually in that case. Additionally the shape of the magnet after upsetting cannot be said a near net shape, which requires finish processing at the entire face, thus leading to disadvantages such as a decrease in material yield and an increase in processing cost due to the addition of processing cost.
When the thickness of a metal capsule covering the entire face of a compact completely is made thinner to be the degree of thickness that is not disclosed in the conventional techniques, such a capsule will be broken at the rate of strain of 1/sec or more, which causes discontinuous unevenness at the rare-earth magnet processed and so causes a disturbance of orientation. In this way, such a method hardly expects high remanence.
Then instead of upsetting that has been used typically, a method of using extruding for hot deformation processing may be considered, so as to give strain to a compact.
For instance, Patent Literature 7 discloses a method for extruding, in which the dimension in X-direction of the extruded cross section at a permanent magnet that is extruded from a pre-compact for shaping is narrowed, whereas the dimension in Y-direction orthogonal thereto is expanded, so that the ratio of strain ε2/ε1 is in the range of 0.2 to 3.5 where ε1 denotes a strain in the extrusion direction at the permanent magnet with reference to the pre-compact, and ε2 denotes a strain in Y-direction. While conventional extruding is typically to get an annular shape, the method disclosed in Patent Literature 7 is to extrude to have a sheet-formed shape.
That is, this method aims to increase the degree of orientation by controlling the stretching in the compression direction and in the direction perpendicular thereto. In order to practically control the stretching in these orthogonal directions precisely, the forming mold has to have a complicated shape, meaning an increase in cost for equipment. Additionally, although extruding can introduce a uniform strain in the travelling direction, it has a large friction area with the forming mold, and so the product obtained tends to have an area with low strain at its center. This is because extruding enables processing by giving compression and shear only, and so cracks due to tension can be suppressed, conversely meaning that the surface of the extruded product becomes a high-strain area because it always receives friction and the center becomes a low-strain area.
Furthermore such extruding requires a forming mold made of a material having high strength at high temperatures because a force at about 200 MPa acts thereon at a temperature near 800° C. when crystals of a Nd—Fe—B rare-earth magnet, for example, are to be oriented by hot deformation processing. For instance, Inconel or, carbide is preferable as such a material of the forming mold, but these carbide metals are difficult to cut, meaning a large burden on the processing cost. When extruding is performed to get a sheet form as in the technique disclosed in Patent Literature 7, stress will be concentrated at the corners of the extruded product because of such a shape, as compared with an annular extruded product. In such a case, the durability of the forming mold will deteriorate, and so the number of products that can be produced with one forming mold will be decreased, which also becomes a factor to increase the processing cost. Although the technique disclosed in Patent Literature 7 aims to improve the performance of the processed product, the shape of extruding is actually complicated three-dimensionally, and so the processing is enabled only with separated molds, and an increase in processing cost is large.
In this way, the development of a method for manufacturing a rare-earth magnet through hot deformation processing is needed so that the rare-earth magnet produced has favorable strains at the entire area and has high degree of orientation and so high remanence without increasing processing cost therefor.